1. Field of the Invention
The present invention relates to a method for determination of the presence of cancer cells in a biological sample, to a method for determination of the prognosis of a patient having cancer, specifically colorectal cancer, and to marker genes used for these methods.
2. Description of the Related Art
Genomic DNAs of higher eukaryotes may sometimes undergo methylation in the 5-position of C (cytosine) among other bases constituting DNAs. Such DNA methylation in higher eukaryotes functions as a mechanism for suppression of expression of genetic information. For example, when a region containing many CpGs (CpG islands), which is often found in promoter regions of certain genes, is methylated, transcription of these genes may be suppressed. On the other hand, when a CpG island is not methylated, a transcription factor can bind to the promoter region and the gene can be transcribed.
Accordingly, DNA methylation is one of control mechanisms of gene expression. DNA methylation plays important roles in various physiological and pathological phenomena such as early embryonic development, expression of tissue specific genes, genomic imprinting and X chromosome inactivation which are characteristic to mammals, stabilization of chromosomes, synchronization of DNA replication and the like.
It has been recently revealed that gene silencing due to DNA methylation is involved in cancer development and progression (see Feinberg A P. and Tycko B., Nat Rev Cancer, Vol. 4, 143-153 (2004); and Jones P A. and Baylin S B., Cell, Vol. 128, 683-692 (2007)).
In the medical field, in order to decide therapeutic strategy for cancer, not only early detection of cancer but also prediction on possibilities of post-operative cancer recurrence or metastasis or on post-operative survival rate of patients for a predetermined period is important; thus it is important to establish a method for determination of prognosis. Prognosis has been conventionally determined based on the evaluation on differentiation states of tumor tissues obtained by operations or biopsies, although it has been unknown whether or not differentiation states is an independent prognosis factor. In addition, histological determination of differentiation states relies on subjective decisions by observers, making the prognosis determination inaccurate.
Especially in the case of colorectal cancer which is mainly represented by well-differentiated adenocarcinoma rather than moderately- or poorly-differentiated adenocarcinoma, prognosis determination may take little account of histology itself. Prognosis determination of colorectal cancer has been difficult because differentiation states of some cases are difficult to be evaluated, making histological diagnosis ambiguous (see Japanese Unexamined Patent Publication Nos. 2001-165933 and 2005-69846).